Tested these and they work fine to charge the Pre with any generic cable.

Apple iPhone 3G AC USB adapter (OEM), 1000ma rating - $3 shipped.

APPLE iPHONE 3G OEM USB POWER ADAPTER CHARGER GREEN DOT - eBay (item 170365561800 end time Aug-27-09 20:23:14 PDT)

NOTE: Takes about 4 sec to start charging - meaning that this adapter is not a candidate to power a Touchstone as this means USB pins 2 and 3 are not bridged inside the charger.

Generic 1000ma Dual-port USB AC adapter - $4.77 shipped

5V 1A Dual 2-Port USB Plug Wall AC Adapter Charger Plug - eBay (item 380141850077 end time Aug-21-09 10:21:35 PDT)

NOTE: This one shows charge indication immediately, meaning pins 2 and 3 are bridged inside the charger, may also thus be able to power Touchstone but not tested with it. Also - can not charge 2 Pre's at once as the 1000ma is apparently the max it can deliver either from a single port or combined.